Traditional printed books, electronic books or other printed media (whether in electronic or physical form) often contain a primary text and additional, supplementary information, such as footnotes, end notes, glossaries and appendices. These items of supplementary information often contain useful or interesting information, such as additional background or explanatory text regarding the primary text, external references, or commentary.
Frequently, printed media are converted into audio format. Generally, this may involve narrating and recording a reading of the printed medium. The resulting audio book or audio content may then be made available to users. Audio content may be played back on a wide variety of devices, such as notebook and tablet computers, mobile phones, personal music players, electronic book (“eBook”) readers, and other audio playback devices or devices with audio playback capability.